Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/102

Rh 82 diatomic; its atomic weight is 58 6, and its symbol Co. Like iron it may be reduced from its oxides by heating with charcoal or in hydrogen gas ; in the former case a small quantity of carbon is retained, forming a substance ana logous to cast-iron. When reduced by hydrogen at a low temperature it forms a black powder which is pyrophoric, or ignites spontaneously in the air, especially if mixed with finely-divided alumina. At a red heat it decomposes water vapour, producing hydrogen and oxide of cobalt. There are two principal oxides. The protoxide, CoO, is obtained as a black powder by calcining the hydrate CoH 2 2. The latter is a red substance obtained by pre cipitation with alkalies from the solution of a cobalt salt. The higher, or sesquioxide, Co 2 O 3, is produced in a hy- drated form from the hydrated protoxide by the action of chlorine, bromine, chloride of lime, or similar oxidizing agents. It may be rendered anhydrous by careful heating, but at a red heat it decomposes, giving off part of its oxygen, and produces a compound analogous in composi tion to magnetic oxide of iron, Fe 3 O 4. The protoxide forms numerous salts, which are usually of a fine rose-red colour. A weak solution of the nitrate or chloride forms the so-called sympathetic ink, which gives a colourless writing when cold, but appears of a bluish- green colour when heated, and fades again on cooling. This effect may be reproduced a great number of tim es if the writing is not too strongly heated, in which case the colour becomes permanent from the formation of a basic salt. With ammonia the oxides of cobalt form a series of compound bases, which give rise to salts of great interest and complexity; these may be regarded as ammonium salts, in which part of the hydrogen is replaced by ammonium and another part by cobalt in various conditions of atomicity corresponding to the oxides. The alloys of cobalt are not of much importance. It combines most readily with arsenic or antimony, forming the highly crystalline compounds known by the general name of speiss, which can scarcely be considered as alloys. With gold and silver it forms brittle compounds, with mercury a silver-white magnetic amalgam. With copper and zinc the alloy is white, resembling the corresponding compounds of the same metals with nickel and manganese. With tin it forms a somewhat ductile alloy of a violet colour. The presence of cobalt in the alloy of copper, zinc, and nickel, known as German silver, is objectionable, as it renders it hard and difficult to roll. The chief use of cobalt in the arts is for the preparation of colours. The protoxide has an intense colouring power when vitrified, and forms the basis of all the blue colours used in glass and porcelain manufacture. The purity of the tint is much affected by traces even of other metallic oxides, especially those of iron, nickel, or copper. Another preparation, known as smalts, is a glass formed by melting cobalt oxide with pure quartz sand and carbonate of potas sium. _ Sometimes the first two substances are subjected to a preliminary heating to produce fritted silicate of a reddish or purple colour, known as za/re, which when fused with the alkaline carbonate in an ordinary glass furnace produces a deep blue glass. This is rendered friable by running it into water, and is then ground between granite millstones, and finally levigated in water. The various products of the levigation are classified into different qualities according to the fineness of the grain and the strength of the colour, the best being those occupying a medium position, the colour diminishing as the fineness of the grain increases. The coarsest variety, known as strewing blue, consisting of rough angular fragments up to about inch diameter, is used for the ground-work of the old-fashioned blue and gold sign boards^ a very effective and durable kind of surface orna mentation. The highest coloured varieties contain from 6 to 7 per cent, of oxide of cobalt. Glass containing only ^J-jj-th part of the oxide is of a distinct blue j with more thai* 18 per cent, it is black. The principal use of smalts is for bluing paper ; it was formerly employed almost exclusively for this purpose, but has now been to a very considerable extent superseded by the use of artificial ultramarine, which is cheaper and ,more easily applied, but is less permanent, as the colour is easily discharged by acids, which is not the case when smalts is used. The pigment known as cobalt blue, used both in oil and water-colour painting, is obtained by mixing the solutions of a cobalt salt and alum, precipitating with ac alkaline carbonate, and strongly heating the gelatinous precipitate of the hydrated oxides of the two metals. Thenard s blue, a phosphate of cobalt and alumina, is pro duced in a similar manner, by precipitation with an alkaline phosphate. Cobalt green, or Rinman s green, is a mixture of the oxides of zinc and cobalt produced from the solu tions of their sulphates by precipitation with carbonate of sodium and ignition. In analysis cobalt is always determined as protoxide, but the separation from the metals with which it is usually associated, especially nickel, is a difficult and tedious operation. Many different processes have been devised, but the most accurate are those of H. Rose and Liebig. The former depends upon the power possessed by chlorine (or bromine) of converting protoxide of cobalt when in solution into sesquioxide, while the corresponding oxide of nickel is not changed. The solution when completely saturated with chlorine is precipitated by carbonate of barium, which carries down the whole of the cobalt as sesquioxide ; the precipitate is redissolved in hydrochloric acid, the whole of the barium salt separated by sulphuric acid, and the cobalt finally precipitated by means of hydrate of potassium. In Liebig s method the oxides of the two metals are heated with cyanide of potassium and boiled, which produces cobalticyanide of potassium, K 2 Co 2 Cy 6 , and cyanide of nickel and potassium, KNiCy 2. By the addition of finely-divided red oxide of mercury the whole of the nickel is precipitated, partly as cyanide and partly as hydrate, while the cobalt compound remains in solution, and is afterwards separated by means of sulphate of copper as cobalticyanide of copper, which is redissolved ; the copper is separated by sulphuretted hydrogen, and the cobalt then obtained as oxide by boiling with caustic potash. The complexity of the composition of the ores, and the high value of the two metals, has led to the application of more refined methods of chemical analysis in their investigation than are required in the assay of the ores of the commoner metals. Plattner s method of dry assay of cobalt and nickel ores is much more rapidly performed than an analysis, and in practised hands is susceptible of considerable accuracy. It depends upon the fact that when a speiss or arsenical compound, containing the four metals iron, cobalt, nickel, and copper is melted with a vitreous flux such as borax in an oxidizing atmosphere, the metals will be oxidized and pass into a slag with the borax in the order indicated above, no cobalt being taken up until the iron has been entirely removed, and similarly the nickel remaining until the cobalt has been completely oxidized. The steps in the process may be easily recognized owing to the difference in the characteristic colour of the oxides, the dark green or black of the iron slag being rendered distinctly blue by the faintest trace of cobalt, and the blue of the latter being similarly affected by nickel, which has a strong brown colouring power. The arsenides of cobalt and nickel, being of a constant composition, are weighed at each step of the process in the proportion of the metal re moved calculated from the difference. Cobalt may be readily detected by the blow-pipe even when in very small